Gas analysis system and method

ABSTRACT

A gas specimen mixture analysis system includes at least one sensor for measuring the concentrations of inert components in the gas specimen mixture, a pressure sensor for measuring the pressure of the gas specimen mixture, and a temperature sensor for measuring the temperature of the gas specimen mixture. A subsystem measures the speed of sound in the gas specimen mixture, and a processing subsystem, responsive to the at least one inert component sensor, the pressure sensor, the temperature sensor, and the subsystem for measuring the speed of sound, is configured to generate a number of sample gas mixtures with varying percentages of hydrocarbon gases, each including the measured inert component concentrations. The processing subsystem is also configured to calculate, for each generated sample gas mixture, the speed of sound therein based on the measured pressure and temperature and the particular percentages of hydrocarbon gases therein, and iteratively compare the measured speed of sound with the calculated speed of sound in different sample gas mixtures until convergence for a particular sample gas mixture. The processing subsystem is further configured to calculate the molecular weight of the particular sample gas mixture and set the molecular weight of the gas specimen mixture to the calculated molecular weight.

FIELD OF THE INVENTION

The subject invention relates to analysis of a gas, such as natural gaswhich is principally made up of hydrocarbon gases, to determine variousproperties of the gas.

BACKGROUND OF THE INVENTION

Various systems and methods exist for determining properties ofmulti-component gases. The gas' heating value, energy flow, mass flowand the like are typical properties of interest. Before a natural gastransmission pipeline company transfers or sells natural gas todistribution companies, the energy content in the natural gas has to bedetermined within a certain accuracy. Also, the energy measurement ispreferred to be on-line during the transfer and in real time.Additionally, a fast and accurate determination of energy in the fuelline of a natural gas turbine would provide for optimal combustion.

Traditionally, energy content in natural gas is calculated from acomplete gas analysis through gas chromatography. Gas chromatography ismostly an “off-line” analysis with a slow response time. Its operationalcost is high due to required consumables and frequent calibrations.

Also, some known systems for determining gas properties without acomplete gas analysis typically make determinations based on limited orfixed, not dynamic, correlations between various components andproperties of the gas. U.S. Pat. Nos. 6,850,847 and 6,216,091, which areeach incorporated herein by reference, are some examples of knownmethods. See, e.g.www.elster-instromet.com/downloads/ELS_GM_EnSonic_DS_UK_ol_l.pdf, whichis incorporated herein by reference.

The challenge is to determine a gas' energy, such as heating value andother properties, more accurately, in real time, and at low cost.

SUMMARY OF THE INVENTION

The present invention provides for more accurate measurement of energycontent in natural gas, for example, among other properties. Themeasurement can be on-line, with fast response time, at a lower cost,and can be used in custody transfer applications and/or for energycontent measurement in a fuel line for a gas turbine, among other uses.Embodiments of this invention provide a cost-effective, less complex,more accurate method and system for determining the molecular weight ofa gas to be measured, thus providing a more accurate determination ofgas properties such as heating value, and energy flow and mass flow.

In the embodiments of this invention, the applicants' method and systemincludes a determination of the molecular weight of a multi-componentgas to be measured as well as various properties of the gas, utilizingthe speed of sound of the gas and its inert components content, such asits nitrogen and carbon dioxide content, when the gas is at any pressureand temperature. These determinations can be made dynamically, and theinvention provides improved accuracy with less complexity and may beincorporated in various conventional field instruments.

The invention embodiments, however, need not achieve all theseobjectives and results and the claims hereof should not be limited tostructures or methods capable of achieving these objectives and results.

This invention features a gas specimen mixture analysis system includingat least one sensor for measuring the concentrations of inert componentsin the gas specimen mixture, a pressure sensor for measuring thepressure of the gas specimen mixture, and a temperature sensor formeasuring the temperature of the gas specimen mixture. The system alsoincludes a subsystem for measuring the speed of sound in the gasspecimen mixture, and a processing subsystem, responsive to the at leastone inert component sensor, the pressure sensor, the temperature sensor,and the subsystem for measuring the speed of sound. The processingsubsystem is configured to generate a number of sample gas mixtures withvarying percentages of hydrocarbon gases but each including the measuredinert component concentrations, calculate, for each generated sample gasmixture, the speed of sound therein based on the measured pressure andtemperature and the particular percentages of hydrocarbon gases therein,iteratively compare the measured speed of sound with the calculatedspeed of sound in different sample gas mixtures until convergence for aparticular sample gas mixture, calculate the molecular weight of theparticular sample gas mixture, and set the molecular weight of the gasspecimen mixture to the calculated molecular weight. In one embodiment,the at least one inert component sensor includes a nitrogen sensor formeasuring the nitrogen gas concentration in the gas specimen mixture anda carbon dioxide sensor for measuring the concentration of carbondioxide in the gas specimen mixture. Convergence may be set to equalityor to a difference between the measured speed of sound and a calculatedspeed of sound less than or equal to 0.001%. The subsystem for measuringthe speed of sound typically includes at least one ultrasonictransducer. The processing subsystem may be further configured togenerate an antecedent sample gas mixture including hydrocarbon gaseseach at percentages which fall within predetermined ranges, and togenerate a subsequent sample gas mixture with at least one hydrocarbongas at a different percentage than in the antecedent sample gas mixturebut still constrained to fall within the predetermined range. Theprocessing subsystem, when the calculated speed of sound in anantecedent sample gas mixture is greater than the measured speed ofsound in the gas specimen mixture, may be configured to generate asubsequent sample gas mixture with percentages of lighter hydrocarbongases decreased from the percentages of lighter hydrocarbon gases in theantecedent sample gas mixture, and/or to generate a subsequent samplegas mixture with percentages of heavier hydrocarbon gases increased fromthe percentages of the heavier hydrocarbon gases in the antecedentsample gas mixture. The processing subsystem, when the calculated speedof sound in an antecedent sample gas mixture is less than the measuredspeed of sound in the gas specimen mixture, may be configured togenerate a subsequent sample gas mixture with percentages of lighterhydrocarbon gases increased from the percentages of lighter hydrocarbongases in the antecedent sample gas mixture, and/or to generate asubsequent sample gas mixture with percentages of heavier hydrocarbongases decreased from the percentages of heavier hydrocarbon gases in theantecedent sample gas mixture.

In a further embodiment the processing subsystem is configured todetermine the heating value of the gas specimen mixture by the stepsincluding converting the molecular weight of the gas specimen mixture toa pure hydrocarbon molecular weight, generating a plurality of samplegas mixtures each having pure hydrocarbon molecular weights, andplotting a correlation curve based on the pure hydrocarbon molecularweights for the plurality of sample gas mixtures and mass-based heatingvalues of the pure hydrocarbon molecular weights for the plurality ofsample gas mixtures, interpolating the mass-based heating value of thepure hydrocarbon molecular weight for the gas specimen mixture from thecorrelation curve, determining the mass-based heating value of the gasspecimen mixture, and calculating the volume-based heating value of thegas specimen mixture using the mass-based heating value of the gasspecimen mixture and density of the gas specimen mixture. The step ofconverting the molecular weight of the gas specimen mixture to a purehydrocarbon molecular weight includes replacing the measured inertcomponent concentrations with proportionately equivalent concentrationsof hydrocarbon gases, and determining the mass-based heating value ofthe gas specimen mixture includes replacing the proportionatelyequivalent concentrations of hydrocarbon gases with the measured inertcomponent concentrations. In one variation, calculating the volume-basedheating value of the gas specimen mixture includes multiplying themass-based heating value of the gas specimen times the density of thegas specimen mixture. The density of the gas specimen mixture is astandard density for the gas specimen mixture based on predeterminedtemperature and pressure values, and the calculated heating value is thevolume-based heating value of the gas specimen mixture at standardtemperature and pressure. The density of the gas specimen mixture is thereal density of the gas specimen mixture based on the measured pressureand temperature, and the calculated heating value is the volume-basedheating value of the gas specimen mixture at the measured temperatureand pressure. In a further embodiment the processing subsystem isconfigured to calculate the energy flow of the gas specimen mixture fromthe volume-based heating value of the gas specimen mixture at themeasured temperature and pressure and volumetric flow rate of the gasspecimen mixture. The volumetric flow rate of the gas specimen mixturemay be measured by a flow meter. Calculating the energy flow of the gasspecimen mixture includes multiplying the volume-based heating value ofthe gas specimen mixture at the measured temperature and pressure timesthe volumetric flow rate.

In another embodiment, the processing subsystem includes determining themass flow rate of the gas specimen mixture by the steps includingmeasuring the volumetric flow rate of the gas specimen mixture,determining the density of the gas specimen mixture, and calculating themass flow of the gas specimen mixture based on the density and thevolumetric flow rate. The volumetric flow rate may be measured by a flowmeter. In one configuration, the step of determining the densityincludes the steps including measuring the temperature and pressure ofthe gas specimen mixture, calculating the density of the particularsample gas mixture from the measured temperature and pressure, andsetting the density of the gas specimen mixture to the calculateddensity. In another configuration, determining the density includes thesteps including calculating the specific gravity of the gas specimenmixture from the molecular weight of the gas specimen mixture, andconverting the calculated specific gravity to density using the measuredinert component concentrations. Calculating the mass flow rate includesmultiplying the density times the volumetric flow rate.

This invention also features a system of analyzing a gas specimenmixture, the system including sensors for measuring the nitrogen gas andcarbon dioxide concentrations of the gas specimen mixture, sensors formeasuring the pressure and temperature of the gas specimen mixture, anda processing subsystem. The processing subsystem is configured togenerate an antecedent sample gas mixture including hydrocarbon gaseseach at a percentage which fall within the predetermined range,calculate the speed of sound in the antecedent sample gas mixture basedon the measured pressure and temperature, measure the speed of sound inthe gas specimen mixture, and compare the measured speed of sound withthe calculated speed of sound and, based on the difference therebetween,generate a subsequent sample gas mixture with at least one hydrocarbongas at a different percentage than in the antecedent sample gas mixturebut still constrained to fall within the predetermined range. Theprocessing subsystem may be further configured to iteratively comparethe measured speed of sound with the calculated speed of sound indifferent generated sample gas mixtures until there is convergence for aparticular sample gas mixture, and to calculate the molecular weight ofthe particular sample gas mixture and set the molecular weight of thegas specimen mixture to the calculated molecular weight. When thecalculated speed of sound in an antecedent sample gas mixture is greaterthan the measured speed of sound in the gas specimen mixture, theprocessing subsystem may be configured to generate a subsequent samplegas mixture with percentages of lighter hydrocarbon gases decreased fromthe percentages of lighter hydrocarbon gases in the antecedent samplegas mixture, and/or when the calculated speed of sound in a sample gasmixture is greater than the measured speed of sound in the gas specimenmixture, the processing subsystem may be configured to generate asubsequent sample gas mixture with percentages of heavier hydrocarbongases increased from the percentages of the heavier hydrocarbon gases inthe antecedent sample gas mixture. When the calculated speed of sound inan antecedent sample gas mixture is less than the measured speed ofsound in the gas specimen mixture, the processing subsystem may beconfigured to generate a subsequent sample gas mixture with percentagesof lighter hydrocarbon gases increased from the percentages of lighterhydrocarbon gases in the antecedent sample gas mixture, and/or when thecalculated speed of sound in an antecedent sample gas mixture is lessthan the measured speed of sound in the gas specimen mixture, theprocessing subsystem may be configured to generate a subsequent samplegas mixture with percentages of heavier hydrocarbon gases decreased fromthe percentages of heavier hydrocarbon gases in the antecedent samplegas mixture.

In one embodiment, the processing subsystem is further configured todetermine the heating value of the gas specimen mixture by the stepsincluding converting the molecular weight of the gas specimen mixture toa pure hydrocarbon molecular weight, generating a plurality of samplegas mixtures each having pure hydrocarbon molecular weights, plotting acorrelation curve based on the pure hydrocarbon molecular weights forthe plurality of sample gas mixtures and mass-based heating values ofthe pure hydrocarbon molecular weights for the plurality of sample gasmixtures, interpolating the mass-based heating value of the purehydrocarbon molecular weight for the gas specimen mixture from thecorrelation curve, determining the mass-based heating value of the gasspecimen mixture, and calculating the volume-based heating value of thegas specimen mixture using the mass-based heating value of the gasspecimen mixture and density of the gas specimen mixture. In oneexample, converting the molecular weight of the gas specimen mixture toa pure hydrocarbon molecular weight includes replacing the measurednitrogen gas and carbon dioxide concentrations with proportionatelyequivalent concentrations of hydrocarbon gases, and determining themass-based heating value of the gas specimen mixture includes replacingthe proportionately equivalent concentrations of hydrocarbon gases withthe measured nitrogen gas and carbon dioxide concentrations. Calculatingthe volume-based heating value of the gas specimen mixture includesmultiplying the mass-based heating value of the gas specimen times thedensity of the gas specimen mixture. The density of the gas specimenmixture may be a standard density for the gas specimen mixture based onpredetermined temperature and pressure values, and the calculatedheating value is the volume-based heating value of the gas specimenmixture at standard temperature and pressure. Alternatively, the densityof the gas specimen mixture may be the real density of the gas specimenmixture based on the measured pressure and temperature, and thecalculated heating value is the volume-based heating value of the gasspecimen mixture at the measured temperature and pressure.

In another embodiment, the processing subsystem is further configured tocalculate the energy flow of the gas specimen mixture from thevolume-based heating value of the gas specimen mixture at the measuredtemperature and pressure and volumetric flow rate of the gas specimenmixture. The volumetric flow rate of the gas specimen mixture may bemeasured by a flow meter, and calculating the energy flow of the gasspecimen mixture may include multiplying the volume-based heating valueof the gas specimen mixture at the measured temperature and pressuretimes the volumetric flow rate.

In a further embodiment, the system in which the processing subsystem isfurther configured to determine the mass flow rate of the gas specimenmixture by the steps including measuring the volumetric flow rate of thegas specimen mixture, determining the density of the gas specimenmixture, and calculating the mass flow of the gas specimen mixture basedon the density and the volumetric flow rate. The volumetric flow ratemay be measured by a flow meter. The step of determining the density mayinclude the steps including measuring the temperature and pressure ofthe gas specimen mixture, calculating the density of the particularsample gas mixture from the measured temperature and pressure, andsetting the density of the gas specimen mixture to the calculateddensity. Alternatively, determining the density may include the steps ofcalculating the specific gravity of the gas specimen mixture from themolecular weight of the gas specimen mixture, and converting thecalculated specific gravity to density using the measured nitrogen gasand carbon dioxide concentrations. Calculating the mass flow rateincludes multiplying the density times the volumetric flow rate.

This invention further features a gas specimen mixture analysis systemincluding at least one sensor for measuring the concentration of inertcomponents in the gas specimen mixture, a pressure sensor for measuringthe pressure of the gas specimen mixture, a temperature sensor formeasuring the temperature of the gas specimen mixture, a subsystem formeasuring the speed of sound in the gas specimen mixture, and aprocessing subsystem. The processing subsystem is responsive to the atleast one inert component sensor, the pressure sensor, the temperaturesensor, and the subsystem for measuring the speed of sound, andconfigured to generate a number of sample gas mixtures with varyingpercentages of hydrocarbon gases but each including the measured inertcomponent concentrations, calculate, for each generated sample gasmixture, the speed of sound therein and molecular weight based on themeasured pressure and temperature and the particular percentages ofhydrocarbon gases therein, generate an interrelationship between thecalculated speed of sound and molecular weight for each generated samplegas mixture, and set the molecular weight of the gas specimen mixturebased on the interrelationship using the measured speed of sound in thegas specimen mixture. The at least one inert component sensor mayinclude a nitrogen sensor for measuring the nitrogen gas concentrationin the gas specimen mixture and a carbon dioxide sensor for measuringthe concentration of carbon dioxide in the gas specimen mixture. Theprocessing subsystem may also be configured to plot a correlation curvebased on the calculated speed of sound and molecular weight for eachgenerated sample gas mixture, and to set the molecular weight of the gasspecimen mixture by interpolating from the correlation curve using themeasured speed of sound in the gas specimen mixture. In anothervariation, the processing subsystem may be configured to formulate anequation based on the calculated speed of sound and molecular weight foreach generated sample gas mixture, and to set the molecular weight ofthe gas specimen mixture by solving the equation using the measuredspeed of sound in the gas specimen mixture. The hydrocarbon gases in thesample gas mixtures may be methane, ethane, propane, butane, pentane,hexane, heptane, octane, nonane, and/or decane. The subsystem formeasuring the speed of sound typically includes at least one ultrasonictransducer.

In one embodiment, the processing subsystem is configured to determinethe heating value of the gas specimen mixture by the steps includingconverting the molecular weight of the gas specimen mixture to a purehydrocarbon molecular weight, generating a plurality of sample gasmixtures each having pure hydrocarbon molecular weights, and plotting acorrelation curve based on the pure hydrocarbon molecular weights forthe plurality of sample gas mixtures and mass-based heating values ofthe pure hydrocarbon molecular weights for the plurality of sample gasmixtures, interpolating the mass-based heating value of the purehydrocarbon molecular weight for the gas specimen mixture from thecorrelation curve, determining the mass-based heating value of the gasspecimen mixture, and calculating the volume-based heating value of thegas specimen mixture using the mass-based heating value of the gasspecimen mixture and density of the gas specimen mixture. In onevariation, converting the molecular weight of the gas specimen mixtureto a pure hydrocarbon molecular weight includes replacing the measuredinert component concentrations with proportionately equivalentconcentrations of hydrocarbon gases, and determining the mass-basedheating value of the gas specimen mixture includes replacing theproportionately equivalent concentrations of hydrocarbon gases with themeasured inert component concentrations. In one example, calculating thevolume-based heating value of the gas specimen mixture includesmultiplying the mass-based heating value of the gas specimen times thedensity of the gas specimen mixture. The density of the gas specimenmixture may be a standard density for the gas specimen mixture based onpredetermined temperature and pressure values, and the calculatedheating value is the volume-based heating value of the gas specimenmixture at standard temperature and pressure. The density of the gasspecimen mixture may be the real density of the gas specimen mixturebased on the measured pressure and temperature, and the calculatedheating value is the volume-based heating value of the gas specimenmixture at the measured temperature and pressure. In another embodiment,the processing subsystem is further configured to calculate the energyflow of the gas specimen mixture from the volume-based heating value ofthe gas specimen mixture at the measured temperature and pressure andvolumetric flow rate of the gas specimen mixture. The volumetric flowrate of the gas specimen mixture may be measured by a flow meter.Calculating the energy flow of the gas specimen mixture includesmultiplying the volume-based heating value of the gas specimen mixtureat the measured temperature and pressure times the volumetric flow rate.

In a further embodiment, the processing subsystem is further configuredto determine the mass flow rate of the gas specimen mixture by the stepsincluding measuring the volumetric flow rate of the gas specimenmixture, determining the density of the gas specimen mixture, andcalculating the mass flow of the gas specimen mixture based on thedensity and the volumetric flow rate. The volumetric flow rate may bemeasured by a flow meter. In one configuration, determining the densityincludes the steps including measuring the temperature and pressure ofthe gas specimen mixture, and calculating the density of the gasspecimen mixture from the measured temperature and pressure. In anotherconfiguration, determining the density includes the steps includingcalculating the specific gravity of the gas specimen mixture from themolecular weight of the gas specimen mixture, and converting thecalculated specific gravity to density using the measured inertcomponent concentrations. Calculating the mass flow rate includesmultiplying the density times the volumetric flow rate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a cross-sectional schematic side view of one example of a gasspecimen mixture analysis system in accordance with the presentinvention;

FIG. 2 is a flow chart depicting the primary steps of one example of aconfiguration of a processing subsystem and an associated method foranalyzing a gas specimen mixture in accordance with the presentinvention;

FIG. 3 is a flow chart depicting examples of additional steps for theprocessing subsystem and method of FIG. 2;

FIG. 4 is a flow chart depicting the primary steps of a further exampleof a configuration of the processing subsystem and an associated methodfor analyzing a gas specimen mixture in accordance with the presentinvention;

FIG. 5 is a flow chart depicting examples of additional steps for theprocessing subsystem and method of FIG. 4;

FIG. 6 is a plot of calculated molecular weight and speed of sound inone example showing how the molecular weight of a gas specimen mixturemay be set in accordance with the present invention;

FIG. 7 is a flow chart depicting the primary steps of one example of aconfiguration of the processing subsystem and an associated method foranalyzing a gas specimen mixture including calculating the heating valuein accordance with the present invention;

FIG. 8 is one example of a plot of molecular weight and mass-basedheating value for a number of generated pure hydrocarbon sample gasmixtures;

FIG. 9 is a flow chart depicting the primary steps of one example of aconfiguration of the processing subsystem and an associated method foranalyzing a gas specimen mixture including calculating the energy flowin accordance with the present invention;

FIG. 10 is a flow chart depicting the primary steps of one example of aconfiguration of the processing subsystem and an associated method foranalyzing a gas specimen mixture including calculating the mass flowrate in accordance with the present invention; and

FIG. 11 is one example of a plot of speed of sound and density for anumber of generated gas samples.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the embodiment or embodiments disclosed below, this inventionis capable of other embodiments and of being practiced or being carriedout in various ways. Thus, it is to be understood that the invention isnot limited in its application to the details of construction and thearrangements of components set forth in the following description orillustrated in the drawings. If only one embodiment is described herein,the claims hereof are not to be limited to that embodiment. Moreover,the claims hereof are not to be read restrictively unless there is clearand convincing evidence manifesting a certain exclusion, restriction, ordisclaimer.

In the field of gas analysis, if the properties of a gas flowing througha pipeline could be determined more precisely, gas could be transferredon the basis of quality rather than quantity, among other benefits. Inorder to make improvements in accuracy, however, there needs to be a wayto better establish the totality of the gas mixture in the pipeline inthe first instance. The subject invention provides an improvedalternative to conventional systems.

In one embodiment of the subject invention, gas specimen mixtureanalysis system 10 includes subsystem 12 for measuring the speed ofsound in gas specimen mixture 14. In one configuration, subsystem 12includes a meter 16 (such as an ultrasonic flowmeter) including at leastone transducer 18, or a pair of transducers 18 and 20, which aretypically ultrasonic transducers. Any of a number of commerciallyavailable GE® ultrasonic flowmeters may be utilized, although theinvention is not so limited.

Although gas specimen mixture 14 may be any multi-component gas flowingthrough pipe or conduit 15, in one example gas specimen mixture 14 is amulti-component gas such as natural gas, e.g. a hydrocarbon dominatedgas, including inert or zero energy content components such as nitrogen22 and carbon dioxide 24, as well as hydrocarbon components such asmethane 26, ethane 28, propane 30, and heavier hydrocarbons 32 such asbutane, pentane and the like, with each of components 22-32 within gasspecimen mixture 14 in varying proportions. As used herein, “inert”components or gases refer to components or gases which have no energycontent.

Gas specimen mixture analysis system 10 further includes nitrogen sensor34 for measuring the concentration of nitrogen gas 22 in gas mixture 14,and carbon dioxide sensor 36 for measuring the concentration of carbondioxide 24 in gas specimen mixture 14. Temperature sensor 38 measuresthe temperature of gas specimen mixture 14, and pressure sensor 40measures the pressure. Nitrogen sensor 34, carbon dioxide sensor 36,temperature sensor 38, and pressure sensor 40 may be any of the varioustypes of such sensors commercially available and known to those skilledin the art. If the inert or zero energy components were components otherthan nitrogen and carbon dioxide, appropriate sensors could be used, andnitrogen and carbon dioxide sensors are not necessary limitations of theinvention. Also, insofar as specific embodiments or examples of thesubject invention describe nitrogen and carbon dioxide as the inert orzero energy level components of a gas specimen mixture to be measured oranalyzed, this is not a necessary limitation of the invention, and themethods and systems of the subject invention apply to any inertcomponents.

Gas analysis system 10 further includes processing subsystem 50 which isresponsive to the inert or zero energy content sensors, e.g. to nitrogengas sensor 34 and carbon dioxide sensor 36, and is also responsive totemperature sensor 38, pressure sensor 40, and subsystem 12. As shown,processing subsystem 50 is included within meter 16, e.g. embedded insoftware in meter 16 which may be an ultrasonic flow meter. This is nota limitation of the invention, however. Processing subsystem 50 andassociated methods may be part of an energy meter or other instrument,embedded in software therein or otherwise included therein as may beknown to those skilled in the art, or may be part of a computer or otherprocessor which may be separate from the remaining systems. Theseexamples are not meant to be limiting, and all or part of the presentinvention may be implemented in a computer such as a digital computer,and/or incorporated in software module(s) and/or computer programscompatible with and/or embedded in conventional ultrasonic flow metersor energy meters, such software module(s) in one example for e.g.gathering data from the various sensors and generating the samples andmaking the computations and the like as described herein, and thecomputer's or meter's main components may include e.g.: a processor orcentral processing unit (CPU), at least one input/output (I/O) device(such as a keyboard, a mouse, a compact disk (CD) drive, and the like),a controller, a display device, a storage device capable of readingand/or writing computer readable code, and a memory, all of which areinterconnected, e.g., by a communications network or a bus. Theprocessing subsystems and methods of the present invention can beimplemented as a computer and/or software program(s) stored on acomputer readable medium in the computer or meter and/or on a computerreadable medium such as a tape or compact disk. The processingsubsystems and methods of the present invention can also be implementedin various meters or a plurality of computers, with the componentsresiding in close physical proximity or distributed over a largegeographic region and connected by a communications network, forexample.

Processing subsystem 50 is configured in accordance with one embodimentof a method of the subject invention, namely, to establish the molecularweight of a gas specimen mixture flowing through a pipe or conduit, suchas a hydrocarbon gas, e.g., natural gas. Thus, processing subsystem 50is configured to generate a number of sample gas mixtures with varyingpercentages of hydrocarbon gases, step 60, FIG. 2. In one example, alist of 2904 hydrocarbon sample gas mixtures is generated with gascompositions as shown in Table 1.

TABLE 1 Methane(C1) Bal. Ethane(C2) 11 points over 0-10% Propane(C3) 11points over 0-4% Butane(C4) 6 points over 0-1% n-Pentane(C5) 2 pointsover 0-0.3% n-Hexane(C6) 2 points over 0-0.2% n-Heptane(C7) 0n-Octane(C8) 0 n-Nonane(C9) 0 n-Decane(C10) 0

The list is based on the Normal Range from AGA (American GasAssociation) Report No. 8, which is incorporated herein by reference,but with two simplifications: (1) C6+ group is replaced by a single C6composition, and (2) isomers are replaced by corresponding single-chainparaffin. The error of +/−0.04% found due to (1), and the error of+/−0.004% found due to (2) were both within acceptable limits.

Each sample gas mixture includes the measured inert gas componentsconcentrations, in this example nitrogen gas and carbon dioxideconcentrations, as measured by nitrogen sensor 34, FIG. 1, and carbondioxide sensor 36. The pressure and the temperature are measured by,e.g. temperature sensor 38, FIG. 1, and pressure sensor 40. For eachgenerated sample gas mixture, processing subsystem 50 calculates thespeed of sound therein based on the measured pressure and temperatureand the particular percentages of hydrocarbon gases therein, step 62,FIG. 2. The speed of sound is a thermodynamic gas property which dependson a gas' pressure and temperature, in this case the pressure andtemperature of the gas specimen mixture in a pipe. In general, speed ofsound (SS) can be expressed as a function of the composition of the gasspecimen mixture, pressure (P) and temperature (T):SS=F(composition of gas specimen mixture,P,T)  (1)

In natural gases, the dominant hydrocarbons are long-chain paraffins(C_(n)H_(2n+2)) as shown for example in Table 2 below, compiled fromdata for 394 GE gas turbine engines, showing normal range and expendedrange for gas turbine application, and normal range for custody transferapplication from the aforementioned AGA Report No. 8. Except formethane, all ranges are from 0% to the percentage indicated.

TABLE 2 Normal Range for Expended Range for Gas Turbine App. Gas TurbineApp. (from 95% of 394 (from 99% of 394 Normal Range turbine data)turbine data) from AGA8 Methane 80%~100%   60% >45%   Ethane   85%   15%<10%   Propane   3%   5% <4% Butane   1%   2% <1% Pentane  0.3%  0.6%<0.3%    Hextane  0.2%  0.4% C6+ total <0.2% Heptane 0.16% 0.32% C6+total <0.2% Octane 0.02% 0.04% C6+ total <0.2% Nonane 0.01% 0.025%  C6+total <0.2% Decane 0.002%  0.008%  C6+ total <0.2% C11~20 tot. 0.001% 0.006%  0 N₂   5%   31% 50% CO₂  4.5%   31% 30% CO   0% 0.02%  3% H₂O0.01% 0.01% 0.05%   Hydrogen 0.02%  0.2% 10% Helium 0.02% 0.06% 0.20%  Argon 0.002%  0.05% 0 Oxgyen 0.04%  0.5% 0 H₂S   0% 0.01% 0.02%  

Because the dominant hydrocarbons are long-chain paraffins, equation (1)can be simplified to:SS=F(MW_(CH),inert components,P,T)  (2a)or if nitrogen and carbon dioxide constitute a majority of the inert orzero energy content components, equation (1) can be simplified to:SS=F(MW_(CH),N₂%,CO₂%,P,T)  (2b)where MW_(CH) is the molecular weight for equivalent pure hydrocarbonsof the gas specimen mixture flowing through a pipe, and N₂% and CO₂% aremole fractions (percentages) of nitrogen and carbon dioxide for the gasspecimen mixture.

After the speed of sound has been calculated for each generated samplegas mixture, processing subsystem 50 iteratively compares the measuredspeed of sound with the calculated speed of sound in different generatedsample gas mixtures, step 64, FIG. 2 until convergence for a particularsample gas mixture, step 66. In one example, processing system 50compares the measured speed of sound with the calculated speed of soundof the sample gas mixture containing 80% methane, 8% ethane, 3% propane,1% butane, 0.3% pentane, and appropriate percentages of remainingcomponents to total 100%. See Table 1. If the measured speed of sound inthe gas specimen mixture does not converge with, the calculated speed ofsound for that particular sample gas mixture, then a different samplegas mixture is selected, e.g. a sample gas mixture containing 81%methane, 7% ethane, 3% propane, 1% butane, 0.3% pentane, and appropriatepercentages of remaining components to total 100%. If there isconvergence between a particular sample gas mixture and the measuredspeed of sound, processing subsystem 50 then calculates the molecularweight of the particular sample gas mixture, step 68, and sets themolecular weight of the gas specimen mixture to the calculated molecularweight, step 70. Convergence may be set to equality between the measuredand calculated speeds of sound, but in one variation, convergence is setto a difference between the measured speed of sound (in the gas specimenmixture) and the calculated speed of sound (in a particular sample gasmixture) of less than or equal to 0.001%.

The molecular weight of the particular sample gas mixture is calculatedusing the measured speed of sound in the gas specimen mixture usingequation (2b), where for example, nitrogen and carbon dioxide are themajor inert gas components. The step of setting the molecular weight ofthe gas specimen mixture to the calculated molecular weight thusprovides the molecular weight of the gas specimen mixture to greaterprecision due to the iterative and dynamic nature of the subjectinvention. The “feedback” loop iteration using the generated sample gasmixtures, with only small incremental differences of concentrations ofhydrocarbon gases therebetween, provides for a finer “tuning”, and muchgreater accuracy is the result.

In one configuration, as part of generating a number of sample gasmixtures including measured inert components, such as nitrogen gas andcarbon dioxide concentration, step 60, processing subsystem 50, FIG. 1generates an antecedent sample gas mixture including hydrocarbon gaseseach at percentages which fall within a predetermined range, step 60 a,FIG. 3. Again referring to Table 1, in one example, the predeterminedrange for methane is between 80%-100%; for ethane between 0%-8%; forpropane between 0%-3%, and so on for the remaining components.

The goal is convergence, in order to calculate the molecular weight ofthe particular gas sample mixture and thus set the molecular weight ofthe gas specimen mixture in the conduit to the calculated molecularweight. Therefore, another particular example of the iterative processis as follows. The antecedent sample gas mixture is generated asdescribed, and the speed of sound of the antecedent sample gas mixtureis calculated, step 62 a, FIG. 3 in the same ways as discussed withrespect to step 62, FIG. 2, and compared with the measured speed ofsound, step 64 a, FIG. 3. When the calculated speed of sound in anantecedent sample gas mixture is greater than the measured speed ofsound in the gas specimen mixture in the pipe, the calculated speed ofsound must be reduced to lead to convergence due to the relationship ofspeed of sound and molecular weight. Thus, processing subsystem 50 maygenerate a subsequent sample gas mixture with percentages of lighterhydrocarbon gases decreased from the percentages of lighter hydrocarbongases in the antecedent sample gas mixture, step 60 c, FIG. 3. As noted,at least one hydrocarbon gas in the subsequent sample gas mixture is ata different percentage than in the antecedent sample gas, although morethan one hydrocarbon gas percentage may be different than in theantecedent sample gas mixture. For example, if the concentration ofmethane in the antecedent sample gas mixture is 90% (see e.g. Table 1)and the calculated speed of sound must be reduced, the methanepercentage in the subsequent sample gas mixture may be decreased to 80%.Alternatively to reduce the calculated speed of sound, processingsubsystem 50 may generate a subsequent sample gas mixture withpercentages of heavier hydrocarbon gases increased from percentages ofheavier hydrocarbon gases in the antecedent sample gas mixture, step 60c′. For example, the percentage of nonane (a heavier hydrocarbon gas) inthe subsequently generated sample gas mixture may be increased.

Conversely, when the calculated speed of sound in an antecedent samplegas mixture is less than the measured speed of sound in the gas specimenmixture, the calculated speed of sound must be increased to lead toconvergence. Thus, processing subsystem 50 may generate a subsequentsample gas mixture with percentages of lighter hydrocarbon gases (e.g.methane, ethane) increased from the percentages of lighter hydrocarbongases in the antecedent sample gas mixture, step 60 d, FIG. 3. Again, atleast one hydrocarbon gas in the subsequent sample gas mixture is at adifferent percentage than in the antecedent sample gas, although morethan one hydrocarbon gas percentage may be different than in theantecedent sample gas mixture. Alternatively to achieve the same end,processing subsystem 50 may generate a subsequent sample gas mixturewith percentages of heavier hydrocarbon gases (e.g. nonane, decane)decreased from percentages of heavier hydrocarbon gases in theantecedent sample gas mixture, step 60 d′. Any combination ofpercentages of hydrocarbon gases, including increasing and decreasingrespective lighter and heavier hydrocarbon gases in antecedent andsubsequent sample gas mixtures, may be used. Also, as noted above, ifthe calculated speed of sound for the generated antecedent sample gasmixture is equal to the measured speed of sound, or the difference isless than or equal to e.g. 0.001%, there would be no need to generate asubsequent sample gas mixture, and the molecular weight could becalculated directly, step 68.

The result is a more accurate evaluation of the quality of the analyzedgas specimen mixture flowing in the pipe, by virtue of the iterativeincremental dynamic process of the subject invention, which leads to anaccurate assessment of molecular weight of the gas specimen mixture, andthus to more accurate determinations of gas properties such as heatingvalue, mass flow, and energy flow, which are discussed in more detailbelow.

In another embodiment of the subject invention, gas specimen mixtureanalysis system 10, FIG. 1 includes the features described aboveincluding the features of processing subsystem 50, and operates in asimilar manner as described above, except that instead of iterativelycomparing the measured speed of sound with the calculated speed of soundin different gas mixtures until convergence for a particular gasmixture, calculating the molecular weight, and setting the molecularweight of the gas specimen mixture to the calculated molecular weight(as shown e.g. in steps 64-70 in FIG. 2), processing subsystem 50 isconfigured as shown in FIG. 4.

Processing subsystem 50 of the embodiment of FIG. 4 is configured inaccordance with another embodiment of a method of the subject invention,where after generating a number of sample gas mixtures, step 60,processing subsystem 50 not only calculates the speed of sound in eachgenerated sample gas mixture based on the measured pressure andtemperature (of the gas specimen mixture flowing in the pipe) and theparticular percentages of hydrocarbon gases therein, but also calculatesthe molecular weight, step 80. Processing subsystem 50 is furtherconfigured to generate an interrelationship between the calculated speedof sound and molecular weight for each generated sample gas mixture,step 82, and using the measured speed of sound in the gas specimenmixture, sets the molecular weight of the gas specimen mixture based onthe interrelationship between the calculated speed of sound andmolecular weight for the generated sample gas mixture, step 84. In onevariation, generating an interrelationship includes plotting acorrelation curve based on the calculated speed of sound and calculatedmolecular weight for each generated sample gas mixture, step 82 a, FIG.5. A sample correlation curve 90 is shown in FIG. 6. Speed of sound isplotted on the x-axis, and molecular weight is plotted on the y-axis. Inthis case, the molecular weight of the gas specimen mixture is set basedby interpolating from correlation curve 90 using the measured speed ofsound in the gas specimen mixture, step 82 aa, FIG. 5. For example, ifthe measured speed of sound is 385 m/s, the molecular weight can beinterpolated from correlation curve 90, FIG. 6, as 19.70 g/mol.

In the example of FIG. 6, correlation curve 90 between speed of soundand molecular weight is for sample gas mixtures at 25° C. and 602 psiawith 5% nitrogen concentration and 2% carbon dioxide concentration, witheach data point representing one sample gas mixture. FIG. 6 furthershows that speed of sound correlates to molecular weight very well,regardless of large variations in the hydrocarbon compositions amongsample gas mixtures. Once the inert content in the gas specimen mixturein a conduit is known, e.g. the nitrogen and carbon dioxideconcentrations, speed of sound is only a function of the molecularweight of the gas specimen mixture at a given temperature and pressure.Correlation curve 90 of FIG. 6 represents a list of seventy-two (72)generated sample gas mixtures, based on the Normal Range compiled fromgas data from 394 GE Gas Turbines, and the list of generated sample gasmixtures were generated with the gas compositions as shown in Table 3.

TABLE 3 Methane(C1) Bal. Erhane(C2)    2%    4% 10% Propane(C3)    1%   2%  5% Butane(C4)  0.5%    2% n-Pentane(C5)  0.1%  0.3% n-Hexane(C6) 0.04%  0.16% n-Heptane(C7)  0.04%  0.16% n-Octane(C8) 0.005% 0.025%n-Nonane(C9) 0.002% 0.006% n-Decane(C10) 0.001% 0.002% N2    5% CO2   2% P (psia) 15 psia 308 psia 602 psia T (° C.) 0° C. 25° C. 50° C.

In this example, the seventy-two (72) sample gas mixtures are created byfull factorial of C2˜C6 (3×3×2×2×2), with various combinations ofC7˜C10, and C1 as the balance, and nitrogen and carbon dioxideconcentrations fixed at 5% and 2% respectively. For each generated gassample mixture, its speed of sound (SS), gas density (ρ),compressibility factor (Z), specific heat at constant pressure (Cp),specific heat at constant volume (Cv), and isentropic ratio (κ) arecalculated for pressures at 15, 308 and 602 psia, and for temperature at0°, 25° and 50° C., respectively, utilizing commercial SonicWaresoftware by Lomic, Inc., which uses Detailed Characterization Methodspublished in AGA (American Gas Association) Report No. 10 and Report No.8, each of which is incorporated herein by reference. All uncertaintieswere found to be well within acceptable limits. Uncertainties due tocurve fit, for example, were found to be mostly within 0.05%. Errors dueto variations in gas composition, for example, were found to be within+/−0.03% for twenty-five gas samples. At various pressures andtemperatures, correlation curves of speed of sound and molecular weightall consistently demonstrated good correlation.

In another variation, generating an interrelationship includesformulating an equation based on the calculated speed of sound andmolecular weight for each generated sample gas mixture, step 82 b, andsetting the molecular weight of the gas specimen mixture by solving theequation using the measured speed of sound in the gas specimen mixture,step 82 bb. One equation reflecting the interrelationship is:MW=0.000119·SS²−0.157271·SS+62.5419  (3)Also as shown in FIG. 6, where the variable MW is the molecular weightof the sample gas mixture in grams per mole and the variable SS is thespeed of sound of the sample gas mixture in meters per second, at 25°C., 602 psia for 5% nitrogen concentration and 2% CO₂ concentration.Also, in the various embodiments of the gas analysis system and methodof the subject invention, the hydrocarbon gases in the sample gasmixtures include some combination of methane, ethane, propane, butane,pentane, hexane, heptane, octane, nonane, and decane, as well as otherhydrocarbon gases which also may be included in the combination.

The gas specimen analysis systems and methods of the present inventionalso may include—in addition to determining the molecular weight of thegas specimen mixture flowing through a pipe—determining the heatingvalue of the gas specimen mixture and/or the energy flow of the gasspecimen mixture, and/or the mass flow rate of the gas specimen mixture.Typically, these determinations will be made utilizing the molecularweight as determined in accordance with the present invention for betteraccuracy.

Processing subsystem 50, FIG. 1 is configured in accordance with anembodiment of another method of the subject invention to determine theheating value of the gas specimen mixture flowing through a pipe. Inthis embodiment processing subsystem 50 determines the heating value byconverting the molecular weight of the gas specimen mixture to a pure,or corresponding, hydrocarbon molecular weight, step 100, FIG. 7 andgenerating a plurality of sample gas mixtures each having purehydrocarbon molecular weights, step 102, with varying percentages ofdifferent hydrocarbon gases within a predetermined range. Typicallythese sample gas mixtures are generated in a similar manner to the othersample gas mixtures discussed herein, but in this case the sample gasmixtures are generated without the measured inert contentconcentrations, such as measured nitrogen and carbon dioxideconcentrations, as discussed more fully below. Next, a correlation curveis plotted based on the pure hydrocarbon molecular weights for theplurality of sample gas mixtures and mass-based heating values of thepure hydrocarbon molecular weights for the plurality of sample gasmixtures, step 104. The mass-based heating values of the purehydrocarbon molecular weights for the generated sample gas mixtures areobtained by the correlation curve, i.e. the mass-based heating value ofthe pure hydrocarbon molecular weight for the gas specimen mixture canbe interpolated from the correlation curve, step 106. As discussedfurther below, the mass-based heating value for the gas specimen mixtureis determined, step 108, and the volume-based heating value of the gasspecimen mixture is calculated using the mass-based heating value of thegas specimen mixture and the density of the gas specimen mixture, step1110.

The mass-based heating value correlation curve is shown in FIG. 8. It isa chemical property correlation between mass-based heating values,H_(m,CH)(BTU/lbm, gross or net), and molecular weight (MW_(CH)) forvarious hydrocarbon compositions within the Normal Range from AGA ReportNo. 8, discussed above. It is a chemical property only related to gascompositions, and independent of process pressure and temperatureconditions. The correlation curve of FIG. 8 is based on a list of 2904generated sample gas mixtures, as discussed above with regard toTable 1. Some uncertainties in H_(m,CH) are introduced by variations ingas compositions which have the same molecular weight, with the largestuncertainty arising from the curve's mid-region. Even in such a case,the uncertainty was found to be +/−0.032% at MW_(CH)=17.655, which iswithin acceptable limits. For a group of twenty-five sample gasmixtures, the uncertainties in H_(m) (net) due to changes in hydrocarboncompositions was found to be within +/−0.04%, which is within anacceptable limit.

As discussed above, in one variation, the molecular weight of the gasspecimen mixture is converted to a pure, or corresponding, hydrocarbonmolecular weight, step 100, by replacing the measured inertconcentrations—nitrogen and carbon dioxide concentrations in oneexample—with proportionately equivalent concentrations of hydrocarbongases. The molecular weight of pure, or corresponding, hydrocarbonsMW_(CH) can be calculated from the molecular weight of the gas specimenmixture MW (as determined according to the system and methods of thepresent invention for increased accuracy) using equation (4) with themeasured nitrogen and carbon dioxide concentrations:

$\begin{matrix}{{MW}_{CH} = \frac{{MW} - {{{MW}_{N_{2}} \cdot N_{2}}\%} - {{{MW}_{{CO}_{2}} \cdot {CO}_{2}}\%}}{1 - {N_{2}\%} - {{CO}_{2}\%}}} & (4)\end{matrix}$

Once MW_(CH) is found, the corresponding mass-based heating value (grossor net) for hydrocarbons, H_(m,CH) can be found from the curve of FIG. 8by interpolation, i.e. step 106 above. After the heating value isinterpolated, it is the heating value for a pure amount of hydrocarbongases. The amount of nitrogen and carbon dioxide concentrations mustthen be taken into account by, conversely, replacing the pure amount ofhydrocarbon gases with these values.

Thus, H_(m,CH) is converted to mass-based heating value with nitrogenand carbon dioxide concentrations included, i.e. H_(m) is determined byequation (5):

$\begin{matrix}{H_{m} = {H_{m,{CH}} \cdot \frac{{MW} - {{{MW}_{N_{2}} \cdot N_{2}}\%} - {{{MW}_{{CO}_{2}} \cdot {CO}_{2}}\%}}{MW}}} & (5)\end{matrix}$

Thus, the step of determining the heating value of the gas specimenmixture, step 108, is achieved by replacing the proportionatelyequivalent concentrations of hydrocarbon gases with the measurednitrogen gas and carbon dioxide concentrations. The mass-based heatingvalue is therefore known, and the heating value of the gas specimenmixture can be calculated by multiplying the mass-based heating value ofthe gas specimen mixture times the density of the gas specimen mixture,step 110, using equation (6):H _(v) =H _(m)·ρ  (6)

If the heating value at standard temperature and pressure is the desiredquantity, then the density of the gas specimen mixture is a standarddensity, ρ₀, for the gas specimen mixture based on predeterminedtemperature and pressure values. In one example, standard temperatureand pressure are 60° F. and 14.73 psia. Alternatively, the heating valueat the measured temperature and pressure may be calculated, step 110,based on the measured temperature and pressure, if the heating value atthe actual measured temperature and pressure is desired. It can be seenthat because heating value is determined ultimately from the molecularweight of the gas specimen mixture, which is found in accordance withthe inventive methods described above, a more precise heating value isobtained for the analyzed gas.

In a further embodiment, processing subsystem 50, FIG. 1 is configuredin accordance with an embodiment of another method of the subjectinvention, to calculate the energy flow of a gas specimen mixtureflowing through the conduit. In this embodiment processing subsystem 50calculates the energy flow from the volume-based heating value of thegas specimen mixture at the measured temperature and pressure, and thevolumetric flow rate of the gas specimen mixture. The volumetric flowrate of the gas specimen mixture in the pipe 15, FIG. 1 is measured bymeter 16 such as an ultrasonic flow meter, step 120, FIG. 9, and thevolume-based heating value of the gas specimen mixture is determined,step 122, in one example by processing subsystem 50 so configured,including steps 100-110, FIG. 7 and as described above. Thus in thisembodiment, the energy flow through the conduit is calculated from thevolume-based heating value and the volumetric flow rate, step 124, FIG.9 and one way of calculating the energy flow is by multiplying thevolume-based heating value H_(v) (BTU/cf) of the gas specimen mixture atthe measured temperature and pressure times the volumetric flow rate Q(cf/sec), step 126. Thus, it can be seen that since energy flow is basedon heating value, and heating value is determined in accordance with theinvention, more precise energy flow information results.

In still a further embodiment, processing subsystem 50, FIG. 1 isconfigured in accordance with another method of the invention, todetermine the mass flow rate of the gas specimen mixture flowing throughthe conduit. In this embodiment processing subsystem 50 determines themass flow rate by measuring the volumetric flow rate of the gas specimenmixture, step 130, FIG. 10, which can be measured by a flow meter, anddetermining the density of the gas specimen mixture, step 132. Thedensity of the gas specimen mixture may be determined in at least one oftwo ways. A first way includes measuring the temperature and pressure ofthe gas specimen mixture, e.g. using temperature sensor 38 and pressuresensor 40, FIG. 1, and calculating the density from the measuredtemperature and pressure. A second way includes calculating the specificgravity of the gas specimen mixture from the molecular weight of the gasspecimen mixture—e.g. the molecular weight determined by the methods andconfiguration of processing subsystem 50 as discussed herein—and usingthe Gross Characterization Method from AGA8, which is incorporatedherein by reference, to calculate density. Once the density isdetermined the mass flow rate of the gas specimen mixture is calculatedbased on the density and volumetric flow rate, step 134, by multiplyingthe density times the volumetric flow rate e.g. by equation (7):m=ρ·Q  (7)where m is mass flow rate, ρ is density, and Q is volumetric flow ratemeasured by a flowmeter.

As noted, the values for density and specific gravity for use indetermining energy flow and/or mass flow rate as set forth above mayobtained in various ways. Standard density may be calculated at oneatmosphere from the equation of state for ideal gas:

$\begin{matrix}{\rho_{0} = \frac{P \cdot {MW}}{R \cdot T}} & (8)\end{matrix}$where ρ₀ is standard density, P=14.73 psia, T=60° F. and R=8.31 J/molK.

Standard density may also be calculated, more accurately, from a gassample with molecular weight using AGA8 Detailed CharacterizationMethod, which is incorporated herein by reference.

If real density is utilized in the determination of energy flow and/ormass flow rate, AGA Report No. 8 publishes a Gross CharacterizationMethod, which is also incorporated herein by reference, for calculatingnatural (hydrocarbon) gas real density at a measured pressure andtemperature, treating natural (hydrocarbon) gas as a mixture of threecomponents: nitrogen, carbon dioxide and hydrocarbons. It uses a virialequation of state model, where compressibility, Z, is expanded as aseries of molar density, with second and third virial coefficients.Virial coefficients are complex functions of composition andtemperature. The Gross Characterization Method utilizes specificgravity, nitrogen and carbon dioxide mole fractions as inputs, andsolves for compressibility (or density) in an iterative process.Specific gravity is the ratio of a gas' density to that of air at aspecific pressure and temperature. If the specified pressure is oneatmosphere, specific gravity is the ratio of molecular weight of the gasto that of air for an ideal gas.

Density at the measured pressure and temperature may also be calculatedfrom a generated sample gas with the same molecular weight by the AGA8Detailed Characterization Method as discussed herein, and which isincorporated herein by reference. In such a case, the density isdetermined by measuring the pressure and temperature of the gas specimenmixture in the conduit, and calculating the density of the particularsample gas mixture from the measured temperature and pressure. Then thedensity of the gas specimen mixture is set to the calculated density.

The real density of a gas specimen mixture in a pipe at measuredtemperature and pressure may also be determined by interpolating from acorrelation curve. FIG. 11 shows an example of such a correlation curve150 for density at 25° C., 602 psia, with 5% nitrogen concentration and2% carbon dioxide concentration. It can be seen from FIG. 11 that if thespeed of sound is measured, the real density may be determined usingcurve 150. FIG. 11 includes data for the seventy-two (72) sample gasmixtures of Table 1. In each of the calculations and determinationsuncertainties arise, such as uncertainties in nitrogen, carbon dioxide,temperature and pressure measurements, and composition variations. Inall instances, however, uncertainties were within acceptable limits, andimproved accuracy was maintained. Moreover, the use of portions of AGA(American Gas Association) Report No. 8 and Report No. 10 are notnecessary limitations of the invention, and other equivalent methodologymay be used with the systems and methods of the present invention.

Accordingly, the various embodiments of the present invention providemore accurate analysis of the properties of a gas flowing through a pipeor conduit system, including a more precise determination of themolecular weight of the gas and information determined therefrom, andthe analysis, measurements and determinations can be achieved in thefield compatibly with existing instruments if desired.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments. Other embodiments will occur to those skilled inthe art and are within the following claims.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed. Those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

1. A system for analyzing a gas specimen mixture comprising: at leastone inert component sensor for measuring the concentrations of inertcomponents in the gas specimen mixture; a pressure sensor for measuringthe pressure of the gas specimen mixture; a temperature sensor formeasuring the temperature of the gas specimen mixture; a subsystem formeasuring the speed of sound in the gas specimen mixture; and aprocessing subsystem, responsive to the at least one inert componentsensor, the pressure sensor, the temperature sensor, and the subsystemfor measuring the speed of sound, and configured to: generate a numberof sample gas mixtures with varying percentages of hydrocarbon gases buteach including the measured inert component concentrations, calculate,for each generated sample gas mixture, the speed of sound therein basedon the measured pressure and temperature and the particular percentagesof hydrocarbon gases therein, iteratively compare the measured speed ofsound with the calculated speed of sound in different sample gasmixtures until convergence for a particular sample gas mixture,calculate the molecular weight of the particular sample gas mixture, andset the molecular weight of the gas specimen mixture to the calculatedmolecular weight.
 2. The system of claim 1 in which the at least oneinert component sensor includes a nitrogen sensor for measuring thenitrogen gas concentration in the gas specimen mixture and a carbondioxide sensor for measuring the concentration of carbon dioxide in thegas specimen mixture.
 3. The system of claim 1 in which convergence isset to a difference between the measured speed of sound and a calculatedspeed of sound less than or equal to 0.001%.
 4. The system of claim 1 inwhich the subsystem for measuring the speed of sound includes at leastone ultrasonic transducer.
 5. The system of claim 1 in which theprocessing subsystem is configured to generate an antecedent sample gasmixture including hydrocarbon gases each at percentages which fallwithin predetermined ranges.
 6. The system of claim 5 in which theprocessing subsystem is configured to generate a subsequent sample gasmixture with at least one hydrocarbon gas at a different percentage thanin the antecedent sample gas mixture but still constrained to fallwithin the predetermined range.
 7. The system of claim 6 in which theprocessing subsystem, when the calculated speed of sound in anantecedent sample gas mixture is greater than the measured speed ofsound in the gas specimen mixture, is configured to generate asubsequent sample gas mixture with percentages of lighter hydrocarbongases decreased from the percentages of lighter hydrocarbon gases in theantecedent sample gas mixture.
 8. The system of claim 7 in which theprocessing subsystem, when the calculated speed of sound in anantecedent sample gas mixture is greater than the measured speed ofsound in the gas specimen mixture, is configured to generate asubsequent sample gas mixture with percentages of heavier hydrocarbongases increased from the percentages of the heavier hydrocarbon gases inthe antecedent sample gas mixture.
 9. The system of claim 6 in which theprocessing subsystem, when the calculated speed of sound in anantecedent sample gas mixture is less than the measured speed of soundin the gas specimen mixture, is configured to generate a subsequentsample gas mixture with percentages of lighter hydrocarbon gasesincreased from the percentages of lighter hydrocarbon gases in theantecedent sample gas mixture.
 10. The system of claim 9 in which theprocessing subsystem, when the calculated speed of sound in anantecedent sample gas mixture is less than the measured speed of soundin the gas specimen mixture, is configured to generate a subsequentsample gas mixture with percentages of heavier hydrocarbon gasesdecreased from the percentages of heavier hydrocarbon gases in theantecedent sample gas mixture.
 11. The system of claim 1 in which theprocessing subsystem is further configured to determine the heatingvalue of the gas specimen mixture by the steps comprising: convertingthe molecular weight of the gas specimen mixture to a pure hydrocarbonmolecular weight; generating a plurality of sample gas mixtures eachhaving pure hydrocarbon molecular weights; plotting a correlation curvebased on the pure hydrocarbon molecular weights for the plurality ofsample gas mixtures and mass-based heating values of the purehydrocarbon molecular weights for the plurality of sample gas mixtures;interpolating the mass-based heating value of the pure hydrocarbonmolecular weight for the gas specimen mixture from the correlationcurve; determining the mass-based heating value of the gas specimenmixture; and calculating the volume-based heating value of the gasspecimen mixture using the mass-based heating value of the gas specimenmixture and density of the gas specimen mixture.
 12. The system of claim11 in which converting the molecular weight of the gas specimen mixtureto a pure hydrocarbon molecular weight includes replacing the measuredinert component concentrations with proportionately equivalentconcentrations of hydrocarbon gases.
 13. The system of claim 12 in whichdetermining the mass-based heating value of the gas specimen mixtureincludes replacing the proportionately equivalent concentrations ofhydrocarbon gases with the measured inert component concentrations. 14.The system of claim 11 in which calculating the volume-based heatingvalue of the gas specimen mixture includes multiplying the mass-basedheating value of the gas specimen times the density of the gas specimenmixture.
 15. The system of claim 14 in which the density of the gasspecimen mixture is a standard density for the gas specimen mixturebased on predetermined temperature and pressure values, and thecalculated heating value is the volume-based heating value of the gasspecimen mixture at standard temperature and pressure.
 16. The system ofclaim 14 in which the density of the gas specimen mixture is the realdensity of the gas specimen mixture based on the measured pressure andtemperature, and the calculated heating value is the volume-basedheating value of the gas specimen mixture at the measured temperatureand pressure.
 17. The system of claim 16 in which the processingsubsystem is further configured to calculate the energy flow of the gasspecimen mixture from the volume-based heating value of the gas specimenmixture at the measured temperature and pressure and volumetric flowrate of the gas specimen mixture.
 18. The system of claim 17 in whichthe volumetric flow rate of the gas specimen mixture is measured by aflow meter.
 19. The system of claim 17 in which calculating the energyflow of the gas specimen mixture includes multiplying the volume-basedheating value of the gas specimen mixture at the measured temperatureand pressure times the volumetric flow rate.
 20. The system of claim 1in which the processing subsystem is further configured to determine themass flow rate of the gas specimen mixture by the steps comprising:measuring the volumetric flow rate of the gas specimen mixture;determining the density of the gas specimen mixture; and calculating themass flow of the gas specimen mixture based on the density and thevolumetric flow rate.
 21. The system of claim 20 in which the volumetricflow rate is measured by a flow meter.
 22. The system of claim 20 inwhich determining the density includes: measuring the temperature andpressure of the gas specimen mixture; calculating the density of theparticular sample gas mixture from the measured temperature andpressure; and setting the density of the gas specimen mixture to thecalculated density.
 23. The system of claim 20 in which determining thedensity includes: calculating the specific gravity of the gas specimenmixture from the molecular weight of the gas specimen mixture, andconverting the calculated specific gravity to density using the measuredinert component concentrations.
 24. The system of claim 20 in whichcalculating the mass flow rate includes multiplying the density timesthe volumetric flow rate.